For a computer and other electronic devices, a DRAM (Dynamic Random Access Memory) is used as a semiconductor memory device capable of performing high-speed operation. A DRAM mainly comprises a memory cell array and a peripheral circuit region for operating the same. A memory cell array comprises a plurality of unit components arranged in a matrix type, and the one unit component comprises one switching transistor and one capacitor.
Like the other semiconductor devices, in order for a DRAM to meet needs for high integration, each cell has been miniaturized, and thus, a flat area needed to form a capacitor has been reduced. Therefore, in order to secure a capacity required for a memory device, the following study has been developed:                Electrodes are formed so as to have a three-dimensional structure.        Upper and lower electrodes are made of metal materials (MIM structure).        Capacitors are made so as to have a high dielectric constant.        
As a result, in a region where minimum processing size (F value) used as a standard indicator of technology level is of 70 nm or less, electrodes of DRAM are essentially required to have a three-dimensional electrode structure. Upper and lower electrodes made of metals have been already commercialized. Therefore, the properties of a capacitor will most likely not be further improved based on such technology. In order to respond to further miniaturization for future, a major investigation to improve the properties of a capacitor is to make a capacitor insulating film having a high dielectric constant as a last solution.
A capacitor of a semiconductor memory device is required to have the properties:
(1) The capacitor has a large capacity, namely, a high dielectric constant (lower EOT, which will be described later); and
(2) The capacitor has a capacitor insulating film with a low leak current.
However, in general, a high dielectric film with a large dielectric constant has low resistance for dielectric breakdown and high leak current. In other words, there is trade off relation between a high dielectric constant and a low leak voltage. In order to realize a more miniaturized memory cell, it is necessary to develop a highly reliable capacitor structure which does not increase a leak current even the capacitor has a high dielectric film, and a method for manufacturing the capacitor.
WO 2009/090979 discloses a method for preventing leak current by using an STO (strontium titanium oxide) film as a high dielectric film and TiN (titanium nitride) as upper and lower electrodes. Specifically, there is disclosed a flat capacitor, in which a buffer electrode made of an amorphous conductor such as TiSiN (titanium silicon nitride) is interposed between a lower electrode and a dielectric, and a dielectric and an upper electrode. WO 2009/090979 discloses that a lower electrode is covered by a buffer electrode of an amorphous conductor, thereby reducing concavity and convexity on the surface of the lower electrode and thus reducing a leak current.
After forming a capacitor, a heat treatment is needed to perform to DRAM at a temperature of 450° C. to 500° C. At this time, since a dielectric film of a single zirconium oxide film cannot have sufficient heat stability, a leak current increases after the heat treatment. Therefore, various attempts have been made in order to improve heat stability. For example, a ZAZ structure (ZrO2/Al2O3/ZrO2; Z means ZrO2 layer, and A means Al2O3 layer) or a laminate in which Al2O3 layers and ZrO2 layers are alternatively laminated, can be used as a multi-layered dielectric film.
Such structures can obtain the desired properties by combining zirconium oxide (ZrO2) having a high dielectric constant and aluminium oxide (Al2O3) having a low dielectric constant but excellent heat stability.
For example, JP2006-135339 discloses a method for forming a multi-layered dielectric film such as an AZ structure, a ZA structure, a ZAZ structure, or a laminate in which ZrO2 thin films and Al2O3 thin films are alternatively laminated used for a DRAM having F of 70 nm or less.